In European Patent application 0 323 937 a gas sensor is described which has a carrier on which the sensor element is applied by means of thin film or even thick film technology. A small plate of electrically insulating material, such as glass, silicon oxide or ceramic, for instance, serves as the carrier for the sensor element. This carrier has rectangular apertures that are so configured that a middle region and an outer region are formed which are connected by strips between elongate apertures. A resistance layer for heating the sensor element is first applied to the middle region of the carrier. Above this resistance layer is an electrically insulating layer deposited thereon. On top of that are two electrodes facing each other across a gap and having a connection to each other through a sensitive layer which also covers them. The electrical contacting of the resistance layer and of the two electrodes of this layering construction is provided over conducting paths deposited on the carrier and passing across the strips connecting the middle and outer regions thereof.
In the gas sensor device described in the above-identified document, arrays of connecting strips are in a straight line configuration. In order to obtain good thermal insulation of the middle region from the outer region of the carrier, the connecting strips must be made very thin, since they are also relatively short. The electrodes of the sensor element above referred to are either rectangular or have dot shaped surfaces. Such sensor elements are of relatively high ohmic resistance, which is a disadvantage with respect to the measurement range of the gas sensor.
It is an object of the present invention to provide a gas sensor of the above described general kind in which the middle portions of the carrier plate which carry a sensor element are better isolated thermally from the outer region of the carrier plate, so that related components and circuits can be mounted in that outer region to better advantage and construction of the device can be simple and efficient.
Briefly, the connecting strips of the carrier plate extending outward from a middle region to the outer region are of a spiral configuration around the middle region supported by them. The sensor element components, two electrodes and a sensitive layer, are applied directly to the upper surface of the middle region of the carrier. The two electrodes are comb shaped and oppose each other in an interdigital configuration defining a meandering gap. The heating element is on the lower surface of the middle region in each case.
The sensor device of the invention has a particularly simple construction. By mounting the heating element on the underside of the mid-region of the carrier and the provision of the electrodes and the sensitive layer of the sensor element directly on the upper surface of the mid-region of the carrier simplifies the manufacture of the sensor device compared with manufacture of the heretofore conventional structure.
The use of the carrier substrate itself as an electrically insulating layer between the heating element and the sensor element is particularly advantageous when a thin ceramic substrate is used, because in that case heat exchange between the front and the back side of the carrier plate takes place very quickly. The provision of the sensor element directly on the upper side of the carrier substrate is also advantageous, because it is then possible to provide not only thick film sensor elements, as for example SnO.sub.2 sensor elements, or, alternatively, thin film sensor elements, such as phthalocyanine sensor elements. The spiral configuration of the connecting strips around the middle region of the carrier plate supported by them produces a particularly good thermal decoupling of the middle region, with its heating element and sensor element, from the outer region of the carrier plate. Since the connecting strips are comparatively long, these can be designed to be substantially more stable and firm than has been the practice heretofore. The spiral structure of the connecting strips also has the advantage of providing a comparatively small space requirement for the sensor device.
The interdigital configuration of the electrodes is advantageous because in this way there is the equivalent of a parallel connection of many resistance elements, so that the aggregate resistance of the sensor layer lying above them is greatly reduced. The aggregate resistance of the sensor element thereby varies under adsorption of noxious gases over a readily accessible resistance range for measurement.
A useful further development of the invention is to locate a temperature-measuring resistance on the underside of a middle region of the carrier plate, in addition to the heating element already mentioned, for providing the necessary exact temperature control for the heating element. It is also useful from the manufacturing point of view to provide the heating element in the form of heating resistors, so that the heating resistances and the temperature measuring resistances can first be printed as closed thick film surfaces and then structurized (cut out) by laser cutting.
With a suitable choice of resistance disposition and heat regulation, a homogenous distribution of temperature affecting the sensing element is obtained.
It is particularly convenient to provide feed-through contacts passing through the mid-region of the carrier plate for connection of the heater and the heat regulating resistance to circuits on the upper side of the carrier. The feed-throughs can be provided, for example, in thick film technology. In that way a substantial simplification of the manufacturing technology is obtained. In the mode of construction according to the invention, the heating power and likewise the loss power of the heat regulation circuit can be kept relatively small. For this purpose it is possible for the heat regulation, with its power dissipating components, and likewise at least part of the evaluation circuit, to be provided on the carrier plate in its outer region, thus providing an advantageous saving of space. The overall construction of at least part of the sensor device of the invention is thus economically and easily handled in thick film and/or thin film technology.
The good thermal decoupling of the gas sensor according to the invention makes possible the integration of many different sensor elements with an evaluation circuit on a carrier substrate. Good thermal decoupling of the individual sensor elements with respect to each other and with respect to the carrier substrate is important for thermodynamic reasons, because the sensor elements change their resistance value by adsorption of noxious gases, as for example CO and NO.sub.x. They are driven at high temperature (about 150.degree. C. to 500.degree. C.) for amplification of the effect just mentioned.
It is advantageous to dispose the completed sensor module, including the sensor elements, the heat regulation and the evaluation circuit, in a casing at least in part sub-divided into chambers, so that the evaluation circuit is undisturbed in a closed-off part of the casing, while every sensor element is located in a ventilated part of the casing. It is particularly advantageous in such a case to guide the gas mixture to be investigated so that it passes through a filter for filtering out special components of the gas mixture to be investigated by the respective sensor elements. A ventilator driven by a motor is advantageously be integrated in the casing, since that makes it possible to guide a gas or particle stream at constant speed over the sensor elements. The possibility of integrating filters into the sensor device construction and, optionally, a ventilator with a ventilation motor, leads to a particularly compact structure of a gas sensor device.